For enriching a component A of a feed gas mixture containing components A and B, an adsorbent material over which component B is more readily adsorbed and component A is less readily adsorbed may be provided, as is disclosed in the prior art. The adsorbent material is typically arranged to contact flow channels in adsorbers or adsorbent beds. When the gas mixture is introduced at a feed pressure and temperature to a first end of the adsorber during a feed step of the process, component B is preferentially adsorbed and a first product enriched in component A may be delivered from the second end of the adsorber as it becomes loaded with component B. The adsorber may then be regenerated to desorb component B in reverse flow so that the process may be repeated cyclically.
Regeneration may be achieved by alternative strategies of pressure swing, displacement purge, thermal swing, or combinations thereof. It has also been claimed that regeneration of an electrically conductive (for example carbon-based) adsorbent material loaded with an adsorbed gas (for example carbon dioxide) may be achieved by applying an electric current in so-called electric swing adsorption.
In pressure swing adsorption (PSA) systems or vacuum pressure swing adsorption systems (VPSA) according to the prior art, the total pressure of the gas contacting the adsorber is reduced (pressure swing) following the feed step, thus reducing the partial pressure of component B contacting the adsorbent, and desorbing component B to be exhausted by purging with a reflux fraction of already enriched component A. The total pressure of the gas mixture in the adsorber is elevated while the gas flow in the adsorber is directed from the first end to the second end thereof, while the total pressure is reduced in the regeneration step while the gas flow in the adsorber is directed from the second end back to the first end. As a result, a “light” product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component A) is delivered from the second end of the adsorber, and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component B) is exhausted from the first end of the adsorber.
Alternatively, the total pressure may be kept approximately constant in the regeneration step, while component B is desorbed by a third preferably less readily adsorbed component C, which was not part of the feed gas mixture, with component C introduced in reverse flow from the second end back to the first end of the adsorbers (displacement purge), thus reducing the partial pressure of component B contacting the adsorbent, and exhausting displaced component B from the first end of the adsorbers. As a result, a first or “light” product (a gas fraction depleted in the more readily adsorbed component B and enriched in the less readily adsorbed component A) is delivered from the second end of the adsorber, and a “heavy” product (a gas mixture including the more strongly adsorbed component B and the displacement component C) is exhausted from the first end of the adsorber.
Regeneration may also be achieved by cyclically raising the temperature (temperature swing) of the adsorbent so as to reduce the adsorptive affinity for all gas species, resulting in desorption of component B which can then be purged in reverse flow by a purge stream either as a reflux of previously enriched component A or by displacement purge with a component C. Thermal swing adsorption (TSA) requires bulk heating and cooling of the adsorbent on a cyclic basis, so has been generally limited to relatively low cycle frequencies in the prior art. The heating step may be achieved by heating the purge stream before admission to the second end of the adsorbers.
Pressure swing and displacement purge may be combined, so that a displacement purge regeneration step is achieved at a lower total pressure than the feed pressure. Similarly, thermal swing may be combined with pressure swing and/or displacement purge regeneration strategies. The distinction of displacement purge processes in the present context is that the displacement purge stream is externally provided and includes a component C that is not contained in the feed gas mixture to be separated, unlike conventional PSA or TSA processes where the purge stream is typically obtained internally as a fraction of the feed gas mixture undergoing separation.
Previously, application of displacement purge processes has been limited by compatibility of components A, B and C. Even within the context of an overall separation being achieved, some intimate mixing will take place due to axial dispersion in the adsorbers, fluid holdup in gas cavities, and leakage across fluid seals and valves. While components B and C must obviously be compatible, as they will be mixed as an intended outcome of the process, cross-contamination between components A and C would also take place to require compatibility of those components as well. Further, the efficient application of known adsorbent materials for performing adsorptive separations utilizing displacement purge-based regeneration techniques has previously been limited due to conventional physical arrangements of the adsorbent material in adsorbers or adsorbent beds, leading to non-optimal separation of some common relatively less-strongly adsorbed feed gas components, particularly when the feed gas also includes another strongly adsorbed component in addition to a desired light product component.
PSA is widely applied in hydrogen purification (e.g. from syngas generated by steam reforming or gasification of a hydrocarbon feedstock, after water gas shifting to minimize carbon monoxide concentration), with components A and B representing hydrogen and carbon dioxide. In that application, displacement purge using air (or any oxygen-containing gas with oxygen appearing as a component C) would in the prior art have been impracticable or at least impractical, owing to the hazards of cross-contamination between hydrogen and oxygen.